In addition to increased abductor hallucis recruitment in ascending and descending stairs, when hip external rotation exercises were added along with toe spreading exercises folks had more recruitment of the vastus medialis (a closed chain external rotator of the leg and thigh)

Keep in mind:

the exercises given were all non weight bearing and open chain for the external rotators. Imagine what might have happened if they were both closed chain AND weight bearing!

They concentrated on the effects of toe spreading (AKAlift/spread/reach) on the abductor hallucis. It also has far reaching effects on the dorsal interossei, long and short extensors of the toes.